A moon halo, also known as a lunar halo, is a captivating optical phenomenon appearing as a large, luminous ring encircling the Moon. This natural display results from the interaction of moonlight with Earth’s atmosphere.
Appearance of Moon Halos
A typical moon halo presents as a distinct, bright ring around the Moon. This circular light often appears whitish, though sometimes faint spectral colors, with red on the inner edge and blue on the outer edge, can be observed. The halo consistently maintains an apparent radius of approximately 22 degrees from the Moon’s center. For a visual estimate, this size is roughly equivalent to the span of an outstretched hand at arm’s length, with the Moon covered by the thumb and the pinky reaching the halo’s edge.
It is important to distinguish moon halos from coronas, another optical phenomenon that can appear around the Moon. Coronas are much smaller and closer to the lunar disk, typically with a radius of around 10 degrees. Unlike halos, coronas are formed by the diffraction of light through water droplets or tiny ice crystals, resulting in a colored disk or series of rings, rather than a large, clear ring.
The Science Behind Moon Halos
Moon halos form when moonlight refracts, or bends, as it passes through millions of tiny ice crystals suspended high in Earth’s atmosphere. These ice crystals are predominantly found within cirrus or cirrostratus clouds, which typically exist at altitudes ranging from 20,000 to 40,000 feet (approximately 6,000 to 12,000 meters).
The ice crystals responsible for these halos commonly possess a hexagonal, column-like structure. When moonlight enters one face of such a crystal and exits another, it is bent at a specific angle. This bending occurs at a minimum deviation angle of approximately 22 degrees, explaining the halo’s consistent size. This phenomenon is primarily one of refraction, creating the luminous ring. The collective effect of countless randomly oriented crystals, each refracting light at this precise angle, forms the complete circular halo observed by an eye on the ground.
Moon Halos and Weather Lore
For centuries, moon halos have been associated with weather predictions, most notably the saying, “ring around the moon means rain soon.” There is a basis for this traditional belief in atmospheric science. The high-altitude cirrus and cirrostratus clouds that produce moon halos often precede the arrival of warm frontal systems. These frontal systems typically bring changes in weather, including precipitation like rain or snow, within the next 24 to 48 hours.
While the presence of a moon halo can indicate atmospheric conditions that might lead to precipitation, it is not a guaranteed predictor of immediate weather change. Cirrus clouds can form and dissipate without an approaching storm system. Therefore, while a moon halo suggests the presence of ice-laden clouds, often precursors to weather shifts, it serves as an indicator of atmospheric conditions rather than a definitive forecast.